In a semiconductor device fabrication process, a plurality of crossing division lines (also referred to as “streets”) are formed on the front side of a substantially disk-shaped semiconductor wafer, such as a silicon (Si) wafer, to thereby partition a plurality of regions where a plurality of devices, such as ICs and LSIs, are respectively formed. The devices are formed in a device area on the front side of the semiconductor wafer.
The semiconductor wafer is separated, e.g., cut, along the division lines to divide the separate regions where the devices are formed, thereby obtaining the individual devices as chips or dies.
This approach is adopted to obtain, for example, individual semiconductor devices, power devices, medical devices, electrical components or MEMS devices from substrates, such as single crystal substrates, glass substrates, compound substrates or polycrystalline substrates, with device areas in which these devices are formed.
If it is desired to maximise the number of devices which can be arranged on the substrate, such as a semiconductor wafer, i.e., the packing density of the devices, the widths of the division lines have to be decreased accordingly. For some substrates, the division line width may be 20 μm or less, e.g., for producing RFID chips or line sensors.
As a method of dividing a substrate, such as a semiconductor wafer, along the division lines, there has been proposed a laser processing method of applying a pulsed laser beam, having a wavelength allowing transmission of the beam through the substrate, to the substrate along the division lines in a condition where a focal point of the pulsed laser beam is located inside the substrate in a subject area to be divided. In this way, a modified layer having a reduced strength is continuously formed inside the substrate along each division line. Subsequently, an external force is applied to the substrate along each division line by using a breaking tool, thereby dividing the substrate into the individual devices as chips or dies. Such a method is disclosed in JP-A-3408805. Further methods in which a modified layer is formed inside a substrate by application of a laser beam and the modified layer is used as a starting point for dividing the substrate are taught in JP-A-2011-171382 and JP-A-2013-055120.
As another method of dividing a substrate, such as a semiconductor wafer, along the division lines, it has been proposed to apply a pulsed laser beam to the substrate in a condition where a focal point of the beam is located at a distance from the front side of the substrate in the direction towards the back side thereof, in order to create a plurality of hole regions in the substrate, such as a single crystal substrate. Each hole region is composed of an amorphous region and a space in the amorphous region open to the front side of the substrate. Subsequently, an external force is applied to the substrate along each division line by using a breaking tool, thus dividing the substrate into the individual devices as chips or dies.
Further, as yet another method of dividing a substrate, there has been proposed a laser processing method of applying a pulsed laser beam, having such a wavelength that it is absorbed by the substrate material, to the substrate along the division lines, so that the substrate is cut by laser ablation.
The fabrication processes referred to above often comprise a grinding step for adjusting the substrate thickness. The grinding step is performed from a back side of the substrate which is opposite to a substrate front side on which the device area is formed.
In particular, in order to achieve a size reduction of electronic equipment, the size of devices, such as semiconductor devices, power devices, medical devices, electrical components, MEMS devices or optical devices, has to be reduced. Hence, substrates having the devices formed thereon are ground in the above grinding step to thicknesses in the low μm range.
However, in known device fabrication processes, problems may arise when a pulsed laser beam, having a wavelength allowing transmission of the beam through the substrate, is applied to the back side of the substrate along the division lines in a condition where the focal point of the pulsed laser beam is located inside the substrate. In this case, the laser beam transmitted through the substrate may be at least partially incident on the devices formed in the device area on the front side of the substrate, thus causing damage to the devices.
Further, when such a pulsed laser beam is applied to the front side of the substrate after the substrate has been ground to a reduced thickness, e.g., in the low μm range, the substrate may be deformed due to substrate expansion caused by the formation of the modified layer inside the substrate. In particular, the substrate may warp, i.e., bend upwards or downwards. Thus, dividing the substrate along the division lines in a straight manner and accurately controlling the position of the focal point of the laser beam in the thickness direction of the substrate is rendered difficult or even entirely unfeasible.
These issues are particularly prominent for the case of substrates with narrow division lines, e.g., division lines having widths of 20 μm or less.
Moreover, in known device fabrication processes in which modified layers or hole regions are formed in a substrate by applying a laser beam thereto, the die strength of chips or dies obtained in the process of dividing the substrate may be reduced. In particular, the application of the laser beam may induce stress in the side walls of the resulting chips or dies, thus lowering the die strength.
The above-identified problems adversely affect the integrity of the chips or dies obtained from a substrate and can result in a significant reduction of device quality.
Hence, there remains a need for an efficient and reliable method of processing a substrate which allows for high quality chips or dies to be obtained.